Many of his writings promoted the Whig Party, an important aspect of British politics from the 17th century to the mid-19th century, and its successor, the Liberal Party. Whigs and Liberals believed the common people had a more positive effect on history than did royalty and that democratic government would bring about steady social progress.[4]

Trevelyan's history is engaged and partisan. Of his Garibaldi trilogy, "reeking with bias", he remarked in his essay "Bias in History", "Without bias, I should never have written them at all. For I was moved to write them by a poetical sympathy with the passions of the Italian patriots of the period, which I retrospectively shared."[4]

Once called "probably the most widely read historian in the world; perhaps in the history of the world."[6] Trevelyan saw how two world wars shook the belief in progress. Historiography has changed and the belief in progress has declined. Historian Roy Jenkins argues:

Trevelyan's reputation as an historian barely survived his death in 1962. He is now amongst the great unread, widely regarded by the professionals of a later generation as a pontificating old windbag, as short on cutting edge as on reliable facts.[7]

What is perhaps most frequently forgotten, or ignored, is the skill of his literary craftsmanship. Trevelyan was a born writer and a natural storyteller; and this, among historians, is a rare gift … If one quality is to be singled out, is should be this, for all historians he is the poet of English history... His work has one other great and enduring merit: the tradition within which it was written. The Victorian liberals and their Edwardian successors have made one of the greatest contributions to science and to culture ever made by a ruling class. To these by birth and by instinct Trevelyan belonged.[8]

Trevelyan made his own reputation by depicting Italian patriot Giuseppe Garibaldi as a great hero who stood for British ideals of liberty. According to David Cannadine:

his great work was his Garibaldi trilogy (1907–11), which established his reputation as the outstanding literary historian of his generation. It depicted Garibaldi as a Carlylean hero—poet, patriot, and man of action—whose inspired leadership created the Italian nation. For Trevelyan, Garibaldi was the champion of freedom, progress, and tolerance, who vanquished the despotism, reaction, and obscurantism of the Austrian empire and the Neapolitan monarchy. The books were also notable for their vivid evocation of landscape (Trevelyan had himself followed the course of Garibaldi's marches), for their innovative use of documentary and oral sources, and for their spirited accounts of battles and military campaigns.[12]

Trevelyan lectured at Cambridge until 1903 at which point he left academic life to become a full-time writer. In 1927 he returned to the University to take up a position as Regius Professor of Modern History, where the single student whose doctorate he agreed to supervise was J. H. Plumb (1936). During his Professorship he was also familiar with Guy Burgess - he gave a positive reference for Burgess when he applied for a post at the BBC in 1935, describing him as a "first rate man", but also stating that "He has passed through the communist measles that so many of our clever young men go through, and is well out of it".[13] In 1940 he was appointed as Master of Trinity College and served in the post until 1951 when he retired.

Shocked by the horrors of the Great War he saw as an ambulance driver just behind the front lines, Trevelyan became more appreciative of conservatism as a positive force, and less insistent that progress was inevitable. In History of England (1926) he searched for the deepest meaning of English history. Cannadine says he reported they were:

the nation's evolution and identity: parliamentary government, the rule of law, religious toleration, freedom from continental interference or involvement, and a global horizon of maritime supremacy and imperial expansion.[14]

Cannadine concluded in G.M. Trevelyan: A Life in History (1992):

During the first half of the twentieth century Trevelyan was the most famous, the most honored, the most influential and the most widely read historian of his generation. He was a scion of the greatest historical dynasty that (Britain) has ever produced. He knew and corresponded with many of the greatest figures of his time... For fifty years, Trevelyan acted as a public moralist, public teacher and public benefactor, wielding unchallenged cultural authority among the governing and the educated classes of his day."

During World War I he commanded a British Red Cross ambulance unit on the Italian front;[15] his defective eyesight meant he was unfit for military service.

Trevelyan was the first president of the Youth Hostels Association and the YHA headquarters are called Trevelyan House in his honour. He worked tirelessly through his career on behalf of the National Trust, in preserving not merely historic houses, but historic landscapes.

England in the Age of Wycliffe, 1368-1520 (1899).[16] His first book, based on his PhD thesis. The title of this work is somewhat misleading, since it concentrates on the political, social and religious conditions of England during the later years of Wiclef's life only. Six of the nine chapters are devoted to the years 1377–1385, while the last two treat the history of the Lollards from 1382 until the Reformation. The work is critical of Roman Catholicism in favor of Wycliffe.[17]

English Social History: A Survey of Six Centuries: Chaucer to Queen Victoria (1942 US and Canada, 1944 UK). ISBN978-0-582-48488-7.[30] Published during the darkest days of World War Two, it painted a nostalgic picture of England's glorious past as the beacon of liberty and progress, stirring patriotic feelings and becoming his best selling book, also his last major history book.

"It is not man's evolution but his attainment that is the greatest lesson of the past and the highest theme of history."

"Let the science and research of the historian find the fact and let his imagination and art make clear its significance."

"Every true history must force us to remember that the past was once as real as the present and as uncertain as the future."

"Since history has no properly scientific value, its only purpose is educative. And if historians neglect to educate the public, if they fail to interest it intelligently in the past, then all their historical learning is valueless except in so far as it educates themselves."